A slice-selective cpmg pulse sequence with a DANTE-Z selective scheme for measuring spatially-resolved T2 distributions.
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1. A method for generating a magnetic resonance pulse sequence for the investigation of a sample by magnetic resonance, comprising:
generating a selective scan;
wherein, the selective scan comprises a first magnetic resonance pulse sequence wherein the magnetization is inverted from z to −z inside a frequency band, and the pulse sequence is represented by [θx−θ±x−τ]n, where 2nθ=180°, with successive RF pulses of rotation angle θ, having identical phases, with delay times τ, repeated n times;
following the selective scan with a cpmg pulse sequence;
generating a non-selective scan;
wherein the non-selective scan comprises generating a second magnetic resonance pulse sequence wherein the magnetization is maintained along z;
applying a magnetic field gradient during the first magnetic pulse sequence, such that a range of frequencies are created in the sample in the direction of the magnetic field gradient;
obtaining a first signal from the selective scan;
obtaining a second signal from the non-selective scan; and,
subtracting the first and second signals to obtain a resulting signal with only selected frequency components for slice selective investigation of the sample.
2. The method of
following the non-selective scan with a cpmg pulse sequence.
3. The method of
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This application claims the benefit of U.S. Provisional Application No. 61/522,978 filed Aug. 12, 2011.
The present invention relates to magnetic resonance imaging in general and spatially resolved magnetic resonance spin-spin relaxation distribution measurements in particular.
A spatially resolved measurement of T2 relaxation (T2 mapping) is one of the most basic magnetic resonance imaging (“MRI”) core analysis measurements employed to determine a wide variety of fluid/matrix properties. MRI employing T2 distribution measurements, including T2 distribution mapping, is an appealing technique for chemical and petroleum engineering, including core analysis, due to its ability to probe the occupancy of pores by water and oil phases [1]. It is suitable, in principle, for studies of a variety of miscible and immiscible processes, including enhanced oil recovery, and for characterizing porous rocks with regard to mass transfer between flowing and stagnant fluids. Recently, it has been recognized to be a promising technique for spatially resolved analysis of the irreducible water saturation of porous rocks [2]. T2 distribution measurements, including T2 distribution mapping, are also widely adopted in clinical applications as well.
It is desirable that a T2 mapping scheme provide as wide an interval of measurable T2 as possible for a comprehensive analysis of relaxation data. Ideally, spatially resolved T2 measurements are expected to give as realistic T2 distributions as regular bulk CPMG measurements. High sensitivity signal-to-noise ratio (“SNR”) measurements are also important since low field magnets are traditionally used for core rock analysis.
Two pulse sequences for one-dimensional (“1-D”) T2 mapping which employ phase encode magnetic resonance imaging techniques, namely CPMG-prepared SPRITE and spin-echo single-point imaging (“SE-SPI”) are described in [3]. The CPMG-prepared SPRITE sequence has no hardware restrictions on the echo timing other than those for a regular CPMG experiment but is relatively slow, as the measurement time is proportional to T2 dimension, and has a worse SNR due to a small radio frequency (“r.f.”) pulse flip angle. The spin-echo SPI provides much faster measurements and with good SNR, but has a restriction on the first echo acquisition time. The latter makes it difficult to measure short T2 (<1 ms) characteristic, e.g., for water in clay-containing rocks and cement-based materials.
Prior art MRI based methods are unable to reliably measure short lifetime signal components in a T2 distribution.
In one implementation, the present disclosure is directed to a method of measuring a signal component in a T2 distribution including using a slice selection method to provide a local T2 measurement through magnetization selected from a specific location through a DANTE-Z method (a known selective excitation method). In a further implementation, the signal component includes more than one signal component and which are short lived signal components.
In another implementation, the present disclosure is directed to a method for generating a magnetic resonance pulse sequence for the investigation of a sample by magnetic resonance, including generating a selective scan; and generating a non-selective scan.
In another implementation, the present disclosure is directed to an MRI based method for measuring spatially-resolved T2 distributions including providing a sample to be imaged; using a slice selection method, providing a local T2 measurement through magnetization selected from a specific location in the sample through a selective excitation method.
For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:
Certain embodiments of the invention include a slice-selective CPMG pulse sequence with a DANTE-Z selective scheme for measuring spatially-resolved T2 distributions. This is also referred to by the inventors as DANTE-Z-CPMG.
Certain embodiments of the invention include a method of measuring spatially-resolved T2 which is based on a slice selective CPMG sequence. Only T2 at a particular spatial position is measured at a time. There are a number of applications where only monitoring of T2 at particular locations is required. For example, if a sample has well-defined large-scale heterogeneities, only T2 relaxation someplace in those heterogeneous regions may be of interest. Another case is when one monitors T2 change during a long repetitive experiment (e.g., a core flood experiment) and, to avoid processing a large amount of data, analyzes T2 relaxation only from few chosen locations.
The DANTE-Z [4; 5] method is based on a selective inverting a DANTE sequence [6], a series of short pulses which act as an effective 180° pulse upon a band of resonance frequencies. Since the magnetization selected by this pulse train lies along z axis, it is affected only by T1 relaxation while waiting for eddy currents to settle after a slice gradient, which is advantageous for short T2 samples with T1>T2 (the latter inequality is typical for water in rocks [7]). For the same reason, it does not introduce any phase problems when combined with another pulse sequence, which may take place for the selective excitation with a global flip angle of 90° when two transverse components of magnetization are created. Besides, it is easy to program and can be implemented on “routine spectrometers, without the need for sophisticated hardware”[8].
Certain embodiments of the invention include supplementing the slice-selective CPMG T2 mapping method with an MRI sequence employing the DANTE-Z selective excitation scheme, in order to visualize the slice of interest. In certain embodiments, a one-time running of such an MRI sequence prior to the slice-selective CPMG helps one to adjust the width and the location of the slice.
The DANTE-Z Selective Excitation Scheme
The DANTE-Z scheme consists of two successive scans: during the first scan, the magnetization is inverted from z to −z within a selected band of resonance frequencies, and during the second scan, it is all maintained along z. Subtracting the signals coming from those scans leaves only selected frequency components in the resulting signal, no matter what happens to the magnetization outside the frequency band after applying DANTE-Z (see
[θ−θ±x−τ]n−αx−Acq± (1)
where 2ηθ=180° and a denotes a pulse that brings the z magnetization to the transverse plane for acquisition or precession purposes. The sequence (1) differs from a basic DANTE pulse train in that the second θ-pulse has been found to cancel all negative sidebands at (2 k+1)/2τ Hz, so that the first (positive) sideband appears only at 1/τ Hz from the central frequency. This diminishes the problem of multiple selective excitation.
DANTE-Z has been shown to yield a good selectivity profile with reduced side lobes as compared to a conventional DANTE sequence with a 90° flip angle [10]. In order to remove the side lobes completely and to improve the profile toward a quasi-rectangular shape, the amplitude of θ pulses is modulated by a sinc function [9; 11]. The resulting profile is shown in
T2 Mapping Sequences
The same DANTE-Z block is used in an auxiliary MRI sequence, which is intended for visualization and adjustment of the slice of interest. In one embodiment of the invention, SPRITE, a purely phase encoding MRI technique, namely its double half k-space (DHK) variant [12], is chosen for this purpose The intrinsic idea of DANTE-Z, to subtract the signals acquired with the alternate storage of the magnetization along ±z axis, well suits SPRITE, where the subtraction is needed when one wants to remove the steady-state component of the SPRITE signal and thereby to make the image intensity be proportional to a prepared magnetization [13]. Thus, no modification is required of the acquisition scheme of SPRITE with prepared magnetization [3; 13]. A diagram of a DANTE-Z/DHK SPRITE sequence according to an embodiment of the present invention is shown in
Test Measurements
Sequences according to the invention were tested on a set of five 2 ml cylindrical vials of water doped with GdCl3, stacked together in a raw of 6 cm long. T2 of water in the vials varied as 19, 48, 88, 143 and 190 ms. The profile of such a composite sample is shown in
Applying the slice gradient of 0.78 G/cm and changing the carrier frequency of 0 pulses from −10 to 10 kHz with respect to the central frequency, individual vials could be selected with good accuracy (
The next experiment was carried out on three sandstone samples—Berea, sandstone #15 and Wallace, all saturated with water, to test the goodness of the DANTE-Z-CPMG sequence at measuring T2 distributions.
For the last test, DANTE-Z-CPMG was applied to water-filled Berea and Bentheimer sandstone core samples that had been partly de-saturated by 3.5-hrs centrifuging at 2000 r.p.m. Profiles of the samples in their initial and de-saturated states are shown in
The tests above show that DANTE-Z-CPMG can probe T2 components down to 0.2 ms. This presents a noticeable advantage over SE-SPI, the previous T2 mapping technique we introduced for porous solids (see
NMR measurements were carried out on an Oxford Instruments Maran DRX spectrometer equipped with a 0.35 T horizontal bore magnet (v=15 MHz), at room temperature. The r.f. probe used was a home-made 54-mm wide birdcage probe with a 90°-pulse duration of 19 μs. The acquisition parameters can be found in the text and figures captions. T2 distributions were measured by using a Laplace inverse transform algorithm UPEN [15].
The natural sandstone samples used in the tests have the following characteristics. Berea: porosity φ=22%, permeability κ=100-200 mD; sandstone #15: φ=16%, κ=7 mD; Wallace: porosity φ=14%, κ=0.1 mD; Bentheimer: porosity φ=24%, κ˜1 D.
The invention can be implemented in a conventional MRI instrument apparatus as a programmed pulse sequence. For example,
Several transmitting elements TA1 to TAn are located in the gradient field system, the entirety of which is also called transmitting antenna means. They surround an object under investigation O and are fed by several independent RF power transmitters TX1 . . . TXn. The RF pulses generated by these RF power transmitters TX1 . . . TXn are determined by the sequence control unit SEQ and triggered at the correct time. The transmitting elements TA1 to TAn irradiate RF pulses onto the object under investigation O located in the volume under investigation V (as described in more detail in
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